3 Preface A major revolution in digital design has taken place over the past decade. Field programmable gate arrays (FPGAs) can now contain over a million equivalent logic gates and tens of thousands of flip-flops. This means that it is not possible to use traditional methods of logic design involving the drawing of logic diagrams when the digital circuit may contain thousands of gates. The reality is that today digital systems are designed by writing software in the form of hardware description languages (HDLs). The most common HDLs used today are VHDL and Verilog. Both are in widespread use. When using these hardware description languages the designer typically describes the behavior of the logic circuit rather than writing traditional Boolean logic equations. Computer-aided design tools are used to both simulate the Verilog or VHDL design and to synthesize the design to actual hardware. This book assumes no previous knowledge of digital design. We use 30 examples to show you how to get started designing digital circuits that you can implement on a Xilinx Spartan3E FPGA using either the Digilent BASYS system board that can be purchased from for $59 or the Digilent Nexys-2 board that costs $99. We will use Active-HDL from Aldec to design, simulate, synthesize, and implement our digital designs. A free student edition of Active-HDL is available from Aldec, Inc. ( To synthesize your designs to a Spartan3E FPGA you will need to download the free ISE WebPACK from Xilinx, Inc. ( The Xilinx synthesis tools are called from within the Aldec Active-HDL integrated GUI. We will use the ExPort utility to download your synthesized design to the Spartan3E FPGA. ExPort is part of the Adept software suite that you can download free from Digilent, Inc. ( A more complete book called Digital Design Using Digilent FPGA Boards Verilog / Active-HDL Edition is also available from Digilent or LBE Books ( This more comprehensive book contains over 75 examples including examples of using the VGA and PS/2 ports. Similar books that use VHDL are also available from Digilent or LBE Books. Many colleagues and students have influenced the development of this book. Their stimulating discussions, probing questions, and critical comments are greatly appreciated. Richard E. Haskell Darrin M. Hanna iii

7 Introduction 1 Introduction Digital Design Using FPGAs The first integrated circuits that were developed in the early 1960s contained less that 100 transistors on a chip and are called small-scale integrated (SSI) circuits. Medium-scale integrated (MSI) circuits, developed in the late 1960s, contain up to several hundreds of transistors on a chip. By the mid 1970s large-scale integrated (LSI) circuits containing several thousands of transistors had been developed. Very-large-scale integrated (VLSI) circuits containing over 100,000 transistors had been developed by the early 1980s. This trend has continued to the present day with 1,000,000 transistors on a chip by the late 1980s, 10,000,000 transistors on a chip by the mid-1990s, over 100,000,000 transistors by 2004, and up to 1,000,000,000 transistors on a chip today. This exponential growth in the amount of digital logic that can be packed into a single chip has produced serious problems for the digital designer. How can an engineer, or even a team of engineers, design a digital logic circuit that will end up containing millions of transistors? In Appendix C we show that any digital logic circuit can be made from only three types of basic gates: AND, OR, and NOT. In fact, we will see that any digital logic circuit can be made using only NAND gates (or only NOR gates), where each NAND or NOR gate contains four transistors. These basic gates were provided in SSI chips using various technologies, the most popular being transistor-transistor logic (TTL). These TTL chips were the mainstay of digital design throughout the 1960s and 1970s. Many MSI TTL chips became available for performing all types of digital logic functions such as decoders, adders, multiplexers, comparators, and many others. By the 1980s thousands of gates could fit on a single chip. Thus, several different varieties of programmable logic devices (PLDs) were developed in which arrays containing large numbers of AND, OR, and NOT gates were arranged in a single chip without any predetermined function. Rather, the designer could design any type of digital circuit and implement it by connecting the internal gates in a particular way. This is usually done by opening up fuse links within the chip using computer-aided tools. Eventually the equivalent of many PLDs on a single chip led to complex programmable logic devices (CPLDs). Field Programmable Gate Arrays (FPGAs) A completely different architecture was introduced in the mid-1980 s that uses RAM-based lookup tables instead of AND-OR gates to implement combinational logic. These devices are called field programmable gate arrays (FPGAs). The device consists of an array of configurable logic blocks (CLBs) surrounded by an array of I/O blocks. The Spartan-3E from Xilinx also contains some blocks of RAM, 18 x 18 multipliers, as well as Digital Clock Manager (DCM) blocks. These DCMs are used to eliminate clock distribution delay and can also increase or decrease the frequency of the clock.

8 2 Introduction Each CLB in the Spartan-3E FPGA contains four slices, each of which contains two 16 x 1 RAM look-up tables (LUTs), which can implement any combinational logic function of four variables. In addition to two look-up tables, each slice contains two D flip-flops which act as storage devices for bits. The basic architecture of a Spartan-3E FPGA is shown in Fig. 1. CLB CLB LUT FF LUT FF Slice Slice LUT FF LUT FF LUT FF LUT FF Slice Slice LUT FF LUT FF CLB CLB IOBs Figure 1 Architecture of a Spartan-3E FPGA The BASYS board from Digilent contains a Xilinx Spartan3E-100 TQ144 FPGA. This chip contains 240 CLBs arranged as 22 rows and 16 columns. There are therefore 960 slices with a total of 1,920 LUTs and flip-flops. This part also contains 73,728 bits of block RAM. Half of the LUTs on the chip can be used for a maximum of 15,360 bits of distributed RAM. By contrast the Nexys-2 board from Digilent contains a Xilinx Spartan3E-500 FG320 FPGA. This chip contains 1,164 CLBs arranged as 46 rows and 34 columns. There are therefore 4,656 slices with a total of 9,312 LUTs and flip-flops. This part also contains 368,640 bits of block RAM. Half of the LUTs on the chip can be used for a maximum of 74,752 bits of distributed RAM. In general, FPGAs can implement much larger digital systems than CPLDs as illustrated in Table 1. The column labeled No. of Gates is really equivalent gates as we have seen that FPGAs really don t have AND and OR gates, but rather just RAM look-up tables. (Each slice does include two AND gates and two XOR gates as part of carry and arithmetic logic used when implementing arithmetic functions including addition and

9 Introduction 3 multiplication.) Note from Table 1 that FPGAs can have the equivalent of millions of gates and tens of thousands of flip-flops. Table 1 Comparing Xilinx CPLDs and FPGAs Xilinx Part No. of Gates No. of I/Os No. of CLBs No. of Flip-flops Block RAM (bits) CPLDs 9500 family 800 6, FPGAs Spartan 5,000 40, ,016 Spartan II 15, , , ,556 16,384 57,344 Spartan IIE 23, , ,456 2,082 15,366 32, ,912 Spartan 3 50,000 5,000, ,320 2,280 71,264 73,728 1,916,928 Spartan-3E 100,000 1,600, ,688 1,920 29,505 73, ,552 Virtex 57,906 1,124, ,144 2,076 26,112 32, ,072 Virtex E 71,693 4,074, ,224 1,888 66,504 65, ,968 Virtex-II 40,960 8,388, , ,648 1,040 99,832 73,728 3,096,576 Modern Design of Digital Systems The traditional way of designing digital circuits is to draw logic diagrams containing SSI gates and MSI logic functions. However, by the late 1980s and early 1990s such a process was becoming problematic. How can you draw schematic diagrams containing hundreds of thousands or millions of gates? As programmable logic devices replaced TTL chips in new designs a new approach to digital design became necessary. Computer-aided tools are essential to designing digital circuits today. What has become clear over the last decade is that today s digital engineer designs digital systems by writing software! This is a major paradigm shift from the traditional method of designing digital systems. Many of the traditional design methods that were important when using TTL chips are less important when designing for programmable logic devices. Today digital designers use hardware description languages (HDLs) to design digital systems. The most widely used HDLs are VHDL and Verilog. Both of these hardware description languages allow the user to design digital systems by writing a program that describes the behavior of the digital circuit. The program can then be used to both simulate the operation of the circuit and synthesize an actual implementation of the circuit in a CPLD, an FPGA, or an application specific integrated circuit (ASIC). Another recent trend is to design digital circuits using block diagrams or graphic symbols that represent higher-level design constructs. These block diagrams can then be compiled to produce Verilog or VHDL code. We will illustrate this method in this book. We will use Active-HDL from Aldec for designing our digital circuits. This integrated tool allows you to enter your design using either a block diagram editor (BDE) or by writing Verilog or VHDL code using the hardware description editor (HDE). Once your hardware has been described you can use the functional simulator to produce waveforms that will verify your design. This hardware description can then be synthesized to logic equations and implemented or mapped to the FPGA architecture.

10 4 Introduction We include a tutorial for using Active-HDL in Appendix A. A free student version of Active-HDL is available on their website. 1 We will use Xilinx ISE for synthesizing our VHDL designs. You can download a free version of ISE TM WebPACK TM from the Xilinx website. 2 This WebPACK TM synthesis tool can be run from within the Aldec Active-HDL development environment as shown in the tutorial in Appendix A. The implementation process creates a.bit file that is downloaded to a Xilinx FPGA on the BASYS board or Nexys-2 shown in Fig. 2. The BASYS board is available to students for $59 from Digilent, Inc. 3 This board includes a 100k-gate equivalent Xilinx Spartan3E FPGA (250k-gate capacity is also available), 8 slide switches, 4 pushbutton switches, 8 LEDs, and four 7-segment displays. The frequency of an on-board clock can be set to 25 MHz, 50 MHz, or 100 MHz using a jumper. There are connectors that allow the board to be interfaced to external circuits. The board also includes a VGA port and a PS2 port. The use of these ports are described in a different book. 4 Another more advanced board, the Nexys-2 board, is also available to students for $99 from Digilent. The Nexys-2 board is similar to the BASYS board except that it contains a 500k- or 1200k-gate equivalent Spartan 3E FPGA, a Hirose FX2 interface for additional add-on component boards, 16 MB of cellular RAM, 16 MB of flash memory, a 50 MHz clock and a socket for a second oscillator. The Nexys-2 is ideally suited for embedded processors. All of the examples in this book can be used on both the BASYS board and the Nexys-2 board. The only difference is that you would use the file basys2.ucf to define the pinouts on the BASYS board and you would use the file nexys2.ucf to define the pinouts on the Nexys-2 board. Both of these files are available to download from Table 2 shows the jumper settings you would use on the two boards. (a) (b) Figure 2 (a) BASYS board, (b) Nexys-2 Board Digital Design Using Digilent FPGA Boards Verilog / Active-HDL Edition; available from

11 Introduction 5 Table 1.2 Board Jumper Settings BASYS Boad Nexys-2 Board Set the JP3 jumper to JTAG Set the POWER SELECT jumper to USB Remove the JP4 jumper to select a 50 MHz Set the MODE jumper to JTAG clock Verilog Verilog is based on the C programming language but it is not C. Verilog is a hardware description language that is designed to model digital logic circuits. It simply has the same syntax as the C programming language but the way it behaves is different. In this book we begin by using the Active-HDL block diagram editor to draw logic circuits using basic gates. When you compile these block diagrams Active-HDL will generate the corresponding Verilog code. The block diagram representing your logic circuit can then be used as a module in a higher-level digital design. This higher-level design can then be compiled to produce its corresponding Verilog code. This hierachical block diagram editor will make it easy to design top-level designs. Sometimes it will be easier to design a digital module by writing a Verilog program directly rather than drawing it using gates. When you do this you can still use the block diagram for this module in higher-level designs. We will illustrate this process in many of our examples. Just like any programming language, you can only learn Verilog by actually writing Verilog programs and simulating the designs using a Verilog simulator that will display the waveforms of the signals in your design. This is a good way to learn not only Verilog but digital logic as well. A companion book 5 that uses VHDL instead of Verilog is available from Digilent or More comprehensive Verilog and VHDL books are also available. 6, 7 5 Introduction to Digital Design Using Digilent FPGA Boards Block Diagram / VHDL Examples, LBE Books, Digital Design Using Digilent FPGA Boards Verilog / Active-HDL Edition, LBE Books, Digital Design Using Digilent FPGA Boards VHDL / Active-HDL Edition, LBE Books, 2009.

12 6 Example 1 Example 1 Switches and LEDs In this example we will show the basic structure of a Verilog program and how to write logic equations for 2-input gates. Example 1a will show the simulation results using Aldec Active-HDL and Example 1b will show how to synthesize the program to a Xilinx FPGA on the BASYS or Nexys-2 board. Prerequisite knowledge: None Learned in this Example: Use of Aldec Active-HDL Appendix A 1.1 Slide Switches The slide switches on the BASYS and Nexys-2 boards are connected to pins on the 3.3 V FPGA through a resistor R as shown in Fig R The value of R is 4.7 kω on the BASYS board sw[i] and 10 kω on the Nexys-2 board. When the slide switch is down it is connected to ground and the input sw[i] to the FPGA is read as a logic 0. When the slide switch is up it is connected to 3.3 Figure 1.1 Slide switch connection V and the input sw[i] to the FPGA is read as a logic 1. There are eight slide switches on the BASYS and Nexys-2 boards. The eight pin numbers on the FPGA corresponding to the eight slide switches are given in a.ucf file. The file basys2.ucf shown in Listing 1.1 defines the pin numbers for all I/O on the BASYS board. Note that we have named the slide switches sw[i], i = 0:7, which correspond to the switch labels on the board. We will always name the slide switches sw[i] in our top-level designs so that we can use the basys2.ucf file without change. Because the pin numbers on the Nexys-2 board are different from those on the BASYS board we will use a different file called nexys2.ucf to define the pin numbers on the Nexys-2 board. The names of the I/O ports, however, will be the same for both boards. Therefore, all of the examples in this book can be used with either board by simply using the proper.ucf file when implementing the design. Both of these.ucf files can be downloaded from LEDs A light emitting diode (LED) emits light when current flows through it in the positive direction as shown in Fig Current flows through the LED when the voltage

14 8 Example 1 There are two different ways that an I/O No current no light pin of an FPGA can be used to turn on an LED. V2 The first is to connect the FPGA pin to V2 in Fig. R LED 1.2 and to connect V1 to ground. Bringing the pin (V2) high will then turn on the LED. To turn off the LED the output pin would be brought low. light Current This is the method used for the LEDs ld[7] ld[0] on the BASYS and Nexys-2 boards. V2 The second method is to connect the R LED FPGA pin to V1 in Fig. 1.2 and to connect V2 to Figure 1.2 Turning on an LED a constant voltage. Bringing the pin (V1) low will then turn on the LED. To turn off the LED the output pin would be brought high. This voltage should be equal to V2 to make sure no current flows through the LED. This second method is the method used for the 7- segment displays on the BASYS and Nexys-2 boards. Examples 9 and 10 will show how to display hex digits on the 7-segment displays. V1 > V2 V1 < V2 1.3 Connecting the Switches to the LEDs Part 1 of the tutorial in Appendix A shows how to connect the input switches to the output LEDs using the block diagram editor (BDE) in Active-HDL. The result is shown in Fig Figure 1.3 Connecting the eight switches to the eight LEDs

15 Switches and LEDs 9 Compiling the file sw2led.bde generates the Verilog file sw2led.v shown in Listing 1.2. Alternatively, by selecting the hardware description editor (HDE) the module statement and port declarations are automatically generated but you will need to write your own assign statement. This can lead to the simpler Verilog program shown in Listing 1.3 where we have combined the module statement and port declarations in a single module statement that conforms to the 2001 Verilog standard. This format makes it easier to see the input and output signals. We can also write a single assign statement to replace the two assign statements in Listing 1.2. It is unnecessary to define the intermediate bus BUS7[7:0] and because sw and ld are the same size we don't need to include the [7:0] in the assign statement. Listing 1.2 sw2led.v // Title : sw2led module sw2led (sw,ld) ; // Port declarations // input [7:0] sw; wire [7:0] sw; output [7:0] ld; wire [7:0] ld; // Signal declarations // wire [7:0] BUS7; // Terminals assignment // // ---- Input terminals --- // assign BUS7[7:0] = sw[7:0]; // ---- Output terminals --- // assign ld[7:0] = BUS7[7:0]; endmodule Listing 1.3 sw2led2.v // Title : sw2led2 module sw2led2 ( input wire [7:0] sw, output wire [7:0] ld ) ; assign ld = sw; endmodule In Parts 2 and 3 of the tutorial in Appendix A we show how to synthesize, implement, and download the design to the FPGA board. In summary, the steps you follow to implement a digital design on the BASYS or Nexys-2 board are the following:

16 10 Example 1 1. Create a new project and design name. 2. Using the BDE create a logic diagram. 3. Save and compile the.bde file. 4. Optionally simulate the design (see Example 2). 5. Synthesize the design selecting the Spartan3E family and the 3s100etq144 device for the BASYS board and the 3s500efg320 device for the Nexys-2 board. 6. Implement the design using either basys2.ucf or nexys2.ucf as the custom constraint file. Check Allow Unmatched LOC Constraints under Translate and uncheck Do Not Run Bitgen under BitStream. Select JTAG Clock as the start-up clock under Startup Options. 7. Use ExPort to download the.bit file to the FPGA board. At this point the switches are connected to the LEDs. Turning on a switch will light up the corresponding LED. Problem 1.1 The four pushbuttons on the BASYS and Nexys-2 boards are connected to pins on the FPGA using the circuit shown in Fig The value of R is 4.7 kω on the BASYS board and 10 kω on the Nexys-2 board. When the pushbutton is up the two resistors pull the input down to ground and the input btn(i) to the FPGA is read as a logic 0. When the pushbutton is pressed the input is pulled up to 3.3 V and the input btn(i) to the FPGA is read as a logic 1. Create a.bde file using Active-HDL that will connect the four pushbuttons to the rightmost four LEDs. Compile and implement the program. Download the.bit file to the FPGA board and test it by pressing the pushbuttons. 3.3 V R R btn(i) Figure 1.4 Pushbutton connection

17 2-Input Gates 11 Example 2 2-Input Gates In this example we will design a circuit containing six different 2-input gates. Example 2a will show the simulation results using Aldec Active-HDL and Example 2b will show how to synthesize the program to a Xilinx FPGA on a Digilent board. Prerequisite knowledge: Appendix C Basic Logic Gates Appendix A Use of Aldec Active-HDL 2.1 Generating the Design File gates2.bde Part 4 of the tutorial in Appendix A shows how to connect two inputs a and b to the inputs of six different gates using the block diagram editor (BDE) in Active-HDL. The result is shown in Fig Note that we have named the outputs of the gates the name of the gate followed by an underscore. Identifier names in Verilog can contain any letter, digit, underscore _, or $. The identifier can not begin with a digit or be a keyword. Verilog is case sensitive. The name of this file is gates2.bde. When you compile this file the Verilog program gates2.v shown in Listing 2.1 is generated. We have modified the module statement to conform to the 2001 Verilog standard as described in Example 1. Figure 2.1 Circuit diagram for Example 2

18 12 Example 2 Listing 2.1 gates2.v // Example 2a: gates2 module gates2 ( input wire a, input wire b, output wire and_, output wire nand_, output wire nor_, output wire or_, output wire xnor_, output wire xor_ ) ; assign and_ = b & a; assign nand_ = ~(b & a assign or_ = b a; assign nor_ = ~(b a assign xor_ = b ^ a; assign xnor_ = ~(b ^ a endmodule The logic diagram in Fig. 2.1 contains six different gates. This logic circuit is described by the Verilog program shown in Listing 2.1. The first line in Listing 2.1 is a comment. Comments in Verilog follow the double slash //. All Verilog programs begin with a module statement containing the name of the module (gates2 in this case) followed by a list of all input and output signals together with their direction and type. We will generally use lower case names for signals. The direction of the input and output signals is given by the Verilog statements input, output, or inout (for a bi-directional signal). The type of the signal can be either wire or reg. In Listing 2.1 all of the signals are of type wire which you can think of as a wire in the circuit in Fig. 2.1 where actual voltages could be measured. We will describe the reg type in Example 5. To describe the output of each gate in Fig. 2.1 we simply write the logic equation for that gate preceded by the keyword assign. These are concurrent assignment statements which means that the statements can be written in any order. 2.2 Simulating the Design gates2.bde Part 4 of the tutorial in Appendix A shows how to simulate this Verilog program using Active-HDL. The simulation produced in Appendix A is shown in Fig Note that the waveforms shown in Fig. 2.2 verify the truth tables for the six gates. Also note that two clock stimulators were used for the inputs a and b. By making the period of the clock stimulator for the input a twice the period of the clock stimulator for the input b all four combinations of the inputs a and b will be generated in one period of the input a.

19 2-Input Gates 13 Figure 2.2 Simulation of logic circuit in Fig Generating a Top-Level Design Part 5 of the tutorial in Appendix A shows how to create a top-level design for the gates2 circuit. In order to use the constraint files basys2.ucf or nexys2.ucf described in Example 1 we must name the switch inputs sw[i] and the LED outputs ld[i]. This toplevel design, as created in Part 5 of Appendix A is shown in Fig The module gates2 in Fig. 2.3 contains the logic circuit shown in Fig Note that each wire connected to a bus must be labeled to identify its connection to the bus lines. Figure 2.3 Top-level design for Example 2

20 14 Example 2 Compiling the top-level design shown in Fig. 2.3 will generate the Verilog program shown in Listing 2.2. The inputs are now the two rightmost slide switches, sw[1:0], and the outputs are the six right-most LEDs ld[5:0]. To associate these inputs and outputs with the inputs a and b and the six output in the gates2 module in Fig. 2.1 and Listing 2.1 we use the Verilog instantiation statement gates2 U1 (.a(sw[1]),.and_(ld[5]),.b(sw[0]),.nand_(ld[4]),.nor_(ld[3]),.or_(ld[2]),.xnor_(ld[1]),.xor_(ld[0]) This Verilog instantiation statement begins with the name of the module being instantiated, in this case gates2 from Listing 2.1. This is followed by an arbitrary name for this module in the top-level design. Here we call it U1. Then in parentheses the inputs and outputs in Listing 2.1 are associated with corresponding inputs and outputs in the top-level design in Fig Note that we connect the input a in Listing 2.1 to the input sw[1] on the FPGA board. The input b in Listing 2.1 is connected to sw[0] and the outputs and_, nand_, or_, nor_, xor_, and xnor_ are connected to the corresponding LED outputs ld[5:0]. Follow the steps in the tutorial in Appendix A and implement this design on the FPGA board. Note that when you change the settings of the two right-most slide switches the LEDs will indicate the outputs of the six gates. Listing 2.2 gates2_top.v // Example 2b: gates2_top module gates2_top (sw,ld) ; input wire [1:0] sw; output wire [5:0] ld; gates2 U1 (.a(sw[1]),.and_(ld[5]),.b(sw[0]),.nand_(ld[4]),.nor_(ld[3]),.or_(ld[2]),.xnor_(ld[1]),.xor_(ld[0])

21 Multiple-Input Gates 15 Example 3 Multiple-Input Gates In this example we will design a circuit containing multiple-input gates. We will create a logic circuit containing 4-input AND, OR, and XOR gates. We will leave it as a problem for you to create a logic circuit containing 4-input NAND, NOR, and XNOR gates. Prerequisite knowledge: Appendix C Basic Logic Gates Appendix A Use of Aldec Active-HDL 3.1 Behavior of Multiple-Input Gates The AND, OR, NAND, NOR, XOR, and XNOR gates we x[1] studied in Example 1 had two inputs. The basic definitions hold x[2] AND z for multiple inputs. A multiple-input AND gate is shown in Fig. x[n] The output of an AND gate is HIGH only if all inputs are HIGH. There are three ways we could describe this multipleinput AND gate in Verilog. First we could simply write the Multiple-input AND gate. Figure 3.1 logic equation as. assign z = x[1] & x[2] &... & x[n]; (3.1) Alternatively, we could use the & symbol as a reduction operator by writing assign z = &x; (3.2) This produces the same result as the statement (3.1) with much less writing. Finally, we could use the following gate instantiation statement for an AND gate. and(z,x[1],x[2],...,x[n] (3.3) In this statement the first parameter in the parentheses is the name of the output port. This is followed by a list of all input signals. A multiple-input OR gate is shown in Fig The output of an OR gate is LOW only if all inputs are LOW. Just as with the AND gate there are three ways we can describe this multiple-input OR gate in x[1] x[2] Verilog. We can write the logic equation as OR z. x[n] assign z = x[1] x[2]... x[n]; Figure 3.2 or we can use the symbol as a reduction operator by writing Multiple-input OR gate. assign z = x;

22 16 Example 3 or we can use the following gate instantiation statement for an OR gate. or(z,x[1],x[2],...,x[n] A multiple-input NAND gate is shown in Fig The output of a NAND gate is LOW only if all inputs are HIGH. We can write the logic equation as x[1] x[2]. NAND z assign z = ~(x[1] & x[2] &... & x[n] x[n] or we can use the ~& symbol as a reduction operator by writing assign z = ~&x; Figure 3.3 Multiple-input NAND gate. or we can use the following gate instantiation statement for an OR gate. nand(z,x[1],x[2],...,x[n] A multiple-input NOR gate is shown in Fig The output of a NOR gate is HIGH only if all inputs are LOW. We can write the logic x[1] equation as x[2]. NOR z assign z = ~(x[1] x[2]... x[n] x[n] or we can use the ~ symbol as a reduction operator by writing assign z = ~ x; Figure 3.4 Multiple-input NOR gate. or we can use the following gate instantiation statement for an OR gate. nor(z,x[1],x[2],...,x[n] A multiple-input XOR gate is shown in Fig What is the meaning of this multiple-input gate? Following the methods we used for the previous multiple-input gates we can write the logic equation as. assign z = x[1] ^ x[2] ^... ^ x[n]; or we can use the ^ symbol as a reduction operator by writing x[1] x[2] x[3] x[4] XOR Figure 3.5 Multiple-input XOR gate. z assign z = ^x; or we can use the following gate instantiation statement for an OR gate. xor(z,x[1],x[2],...,x[n] We will create a 4-input XOR gatge in this example to determine its meaning but first consider the multiple-input XNOR gate shown in Fig What is the meaning of this multiple-input gate? (See the problelm at the end of this x[1] x[2] x[n] XNOR Figure 3.6 Multiple-input XNOR gate. z

23 Multiple-Input Gates 17 example for the answer.) Following the methods we used for the previous multiple-input gates we can write the logic equation as. assign z = ~(x[1] ^ x[2] ^... ^ x[n] or we can use the ~^ symbol as a reduction operator by writing assign z = ~^x; or we can use the following gate instantiation statement for an XOR gate. xnor(z,x[1],x[2],...,x[n] 3.2 Generating the Design File gates4.bde Use the block diagram editor (BDE) in Active-HDL to create the logic circuit called gates4.bde shown in Fig A simulation of this circuit is shown in Fig From this simulation we see that the output of an XOR gate is HIGH only if the number of HIGH inputs is ODD. Figure 3.7 Block diagram for gates4.bde If you look at the file gates4.v that is generated when you compile gates4.bde you will see that Active-HDL defines separate modules for the 4-input AND, OR, and XOR gates and then uses a Verilog instantiation statement to "wire" them together. Alternatively, we could use the HDE editor to write the simpler Verilog program called gates4b.v shown in Listing 3.1 that uses reduction operators to implement the three 4-input gates. This Verilog program will produce the same simulation as shown in Fig. 3.8.

25 Multiple-Input Gates 19 Listing 3.2: gates4_top.v // Example 2: 4-input gates - top level module gates4_top ( input wire [3:0] sw, output wire [2:0] ld gates4 U1 (.and4_(ld[2]),.or4_(ld[1]),.x(sw),.xor4_(ld[0]) endmodule Problem 3.1 Use the BDE to create a logic circuit containing 4-input NAND, NOR, and XNOR gates. Simulate your design and verify that the output of an XNOR gate is HIGH only if the number of HIGH inputs is EVEN. Create a top-level design that connects the four inputs to the rightmost four slide switches and the three outputs to the three rightmost LEDs. Implement your design and download it to the FPGA board. 3.2 The circuit shown at the right is for a 2 x 4 decoder. Use the BDE to create this circuit and simulate it using Active-HDL. Choose a counter stimulator for x[1:0] that counts every 20 ns, set en to a forced value of 1, and simulate it for 100 ns. Make a truth table with (x[1], x[0]) as the inputs and y[0:3] as the outputs. What is the behavior of this decoder? x[1] x[0] y[0] y[1] y[2] y[3] en

26 20 Example 4 Example 4 Equality Detector In this example we will design a 2-bit equality detector using two NAND gates and an AND gate. Prerequisite knowledge: Appendix C Basic Logic Gates Appendix A Use of Aldec Active-HDL 4.1 Generating the Design File eqdet2.bde The truth table for a 2-input XNOR gate is shown in Fig Note that the output z is 1 when the inputs x and y are equal. Thus, the XNOR gate can be used as a 1-bit equality detector. x y XNOR z z = ~(x ^ y) z = x ~^ y x y z Figure 4.1 The XNOR gate is a 1-bit equality detector By using two XNOR gates and an AND gate we can design a 2-bit equality detector as shown in Fig Use the BDE to create the file eqdet2.bde using Active- HDL. Figure 4.2 Block diagram of a 2-bit equality detector, eqdet2.bde

28 22 Example 5 Example 5 2-to-1 Multiplexer: if Statement In this example we will show how to design a 2-to-1 multiplexer and will introduce the Verilog if statement. Section 5.1 will define a multiplexer and derive the logic equations for a 2-to-1 multiplexer. Section 5.2 will illustrate the use of two versions of the Verilog if statement. Prerequisite knowledge: Karnaugh Maps Appendix D Use of Aldec Active-HDL Appendix A 5.1 Multiplexers An n-input multiplexer (called a MUX) is an n-way digital switch that switches one of n inputs to the output. A 2-input multiplexer is shown in Fig The switch is controlled by the single control line s. This bit selects one of the two inputs to be "connected" to the output. This means that the logical value of the output y will be the same as the logical value of the selected input. From the truth table in Fig. 5.1 we see that y = a if s = 0 and y = b if s = 1. The Karnaugh map for the truth table in Fig. 5.1 is shown in Fig We see that the logic equation for y is y = ~s & a s & b (5.1) Note that this logic equation describes the circuit diagram shown in Fig ab s a b 2 x 1 MUX s y s a b y y = ~s & a s & b Figure 5.1 A 2-to-1 multiplexer Figure 5.2 K-map for a 2-to-1 multiplexer

30 24 Example The Verilog if statement The behavior of the 2 x 1 multiplexer shown in Fig. 5.1 can be described by the Verilog statements if(s == 0) y = a; else y = b; The Verilog if statement must be cont ained in an always block as shown in Listing 5.2. Note that y must be declared to be of type reg because it is assigned a value within the always block. The in the always statement is equivalent where a, b, s is called the sensitivity list. Any time any of these input values change the if statement within the always block is executed. The use of the * notation is a convenience that prevents you from omitting any of the signals or inputs used in the always block. A Verilog program can contain more than one always blocks, and these always blocks are executed concurrently. The Verilog code in Listing 5.2 will be compiled to produce the logic circuit shown in Fig A simulation of the Verilog code in Listing 5.2 will produce the same waveform as shown in Fig Listing 5.2 Example4b.v // Example 4b: 2-to-1 MUX using if statement module mux21b ( input wire a, input wire b, input wire s, output reg y if(s == 0) y = a; else y = b; endmodule Create a top-level design called mux21_top.bde that connects a and b to the rightmost two slide switches, connects s to btn[0], and connects the output y to ld[0]. Implement your design and download it to the FPGA board. Test the operation of the multiplexer by changing the position of the toggle switches and pressing pushbutton btn[0].

31 Quad 2-to-1 Multiplexer 25 Example 6 Quad 2-to-1 Multiplexer In this example we will show how to design a quad 2-to-1 multiplexer. In Section 6.1 we will make the quad 2-to-1 multiplexer by wiring together four of the 2-to-1 multiplexers that we designed in Example 5. In Section 6.2 we will show how the quad 2-to-1 multiplexer can be designed using a single Verilog if statement. Finally, in Section 6.3 we will show how to use a Verilog parameter to define a generic 2-to-1 multiplexer with arbitrary bus sizes. Prerequisite knowledge: Example 5 2-to-1 Multiplexer 6.1 Generating the Design File mux42.bde By using four instances of the 2-to-1 MUX, mux21.bde, that we designed in Example 5, we can design a quad 2-to-1 multiplexer as shown in Fig Use the BDE to create the file mux24.bde using Active-HDL. Note that you will need to add the file mux21.bde to your project. Figure 6.1 The quad 2-to-1 MUX, mux24.bde, contains four 2-to-1 MUXs

33 Quad 2-to-1 Multiplexer 27 Use the BDE to create the top-level design called mux21_top.bde shown in Fig Note that a[3:0] are connected to the four leftmost slide switches, b[3:0] are connected to the rightmost four slide switches, and y[3:0] are connected to the rightmost LEDs. Also note that s is connected to btn[0], and the input btn[0:0] must be declared as an array, even though there is only one element, so that we can use the constraint file basys2.ucf or nexys2.ucf without change. Implement your design and download it to the FPGA board. Test the operation of the quad 2-to-1 multiplexer by setting the switch values and pressing pushbutton btn[0]. If you compile the file mux24_top.bde Active-HDL will generate the Verilog program mux24_top.v shown in Listing 6.2. A simulation of mux24_top.bde is shown in Fig Figure 6.3 Top-level design for testing the quad 2-to-1 MUX Listing 6.2 Example6b.v // Example 6b: mux24_top module mux24_top ( input wire [0:0] btn; input wire [7:0] sw; output wire [3:0] ld; ) ; mux24 U1 (.a(sw[7:4]),.b(sw[3:0]),.s(btn[0]),.y(ld) endmodule Figure 6.4 Simulation of mux24_top.bde in Fig. 6.1

34 28 Example A Quad 2-to-1 Multiplexer Using an if Statement In Listing 5.2 of Example 5 we used a Verilog if statement to implement a 2-to-1 MUX. Listing 6.3 is a direct extension of Listing 5.2 where now the inputs and outputs are 4-bit values rather that a single bit. The Verilog program shown in Listing 6.3 will produce the same simulation as shown in Fig The module mux24b defined by the Verilog program in Listing 6.3 could be used in place of the mux24 module in the toplevel design in Fig. 6.3 Listing 6.3 mux24b.v // Example 6c: Quad 2-to-1 mux using if statement module mux24b( input wire [3:0] a, input wire [3:0] b, input wire s, output reg [3:0] y if(s == 0) y = a; else y = b; endmodule 6.3 Generic Multiplexers: Parameters We can use the Verilog parameter statement to design a generic 2-to-1 multiplexer with input and output bus widths of arbitrary size. Listing 6.4 shows a Verilog program for a generic 2-to-1 MUX. Note the use of the parameter statement that defines the bus width N to have a default value of 4. This value can be overridden when the multiplexer is instantiated as shown in Listing 6.5 for an 8-line 2-to-1 multiplexer called M8. The parameter override clause is automatically included in the module instantiation statement when you copy it in Active-HDL as shown in Listing 6.5. We will always use upper-case names for parameters. The simulation of Listing 6.5 is shown in Fig If you compile the Verilog program mux2g.v shown in Listing 6.4 it will generate a block diagram for this module when you go to BDE. If you right-click on the symbol for mux2g and select Properties, you can change the default value of the parameter N by selecting the Parameters tab and entering an actual value for N.

36 30 Example 7 Example 7 4-to-1 Multiplexer In this example we will show how to design a 4-to-1 multiplexer. In Section 7.1 we will make a 4-to-1 multiplexer by wiring together three of the 2-to-1 multiplexers that we designed in Example 5. In Section 7.2 we will derive the logic equation for a 4-to-1 MUX. In Section 7.3 we will show how a 4-to-1 multiplexer can be designed using a single Verilog case statement and in Section 7.4 we design a quad 4-to-1 multiplexer. Prerequisite knowledge: Example 5 2-to-1 Multiplexer 7.1 Designing a 4-to-1 MUX Using 2-to-1 Modules A 4-to-1 multiplexer has the truth table shown in Fig. 7.1 By using three instances of the 2-to-1 MUX, mux21.bde, that we designed in Example 5, we can design a 4-to-1 multiplexer as shown in Fig Use the BDE to create the file mux41.bde using Active-HDL. Note that you will need to add the file mux21.bde to your project. s1 s0 z 0 0 c0 0 1 c1 1 0 c2 1 1 c3 Figure 7.1 Truth table for a 4-to-1 MUX Figure 7.2 The 4-to-1 MUX, mux41.bde, contains four 2-to-1 MUXs

37 4-to-1 Multiplexer 31 In Fig. 7.2 when s[1] = 0 it is v, the output of U2 that gets through to z. If s[0] = 0 in U2 then it is c[0] that gets through to v and therefore to z. If s[0] = 1 in U2 then it is c[1] that gets through to v and therefore to z. If, on the other hand, s[1] = 1 in U1 then it is w, the output of U3 that gets through to z. If s[0] = 0 in U3 then it is c[2] that gets through to w and therefore to z. If s[0] = 1 in U3 then it is c[3] that gets through to w and therefore to z. Thus you can see that the circuit in Fig. 7.2 will implement the truth table in Fig When you compile the file mux41.bde Active-HDL will generate the Verilog program mux41.v shown in Listing 7.1. A simulation of mux41.bde is shown in Fig Note that the output z will be one of the four inputs c[3:0] depending on the value of s[1:0]. Listing 7.1 mux41.v // Example 7a: 4-to-1 MUX using module instantiation module mux41 ( input wire [3:0] c, input wire [1:0] s, output wire z // Internal signals wire v; // output of mux M1 wire w; // output of mux M2 // Module instantiations mux21 U1 (.a(v),.b(w),.s(s[1]),.y(z) mux21 U2 (.a(c[0]),.b(c[1]),.s(s[0]),.y(v) mux21 U3 (.a(c[2]),.b(c[3]),.s(s[0]),.y(w) endmodule If you were going to create this top-level design using HDE instead of BDE you would begin by defining the inputs c[3:0] and s[1:0] and the output z and the two wires v and w. You would then wire the three modules together using the three module instantiation statements shown in Listing 7.1. The easiest way to generate this module instantiation statement is to first compile the file mux21.v from Example 5 using Active-HDL, expand the library icon (click the

39 4-to-1 Multiplexer 33 At this point you would have the statement mux21 Label1 (.a(a),.b(b),.s(s),.y(y) Make three copies of this prototype and change the name of Label1 to U1, U2, and U3 in the three statements. Now you just wire up each input and output variable by changing the values in the parentheses to the signal that it is connected to. For example, the mux U1 input a is connected to the wire v so we would write.a(v). In a similar way the mux input b is connected to wire w and the mux input s is connected to input s[1]. The mux output y is connected to the output z in Fig Thus, the final version of this module instantiation statement would be mux21 U1 (.a(v),.b(w]),.s(s[1]),.y(z) The other two modules, U2 and U3, are wired up using similar module instantiation statements. 7.2 The Logic Equation for a 4-to-1 MUX The 4-to-1 MUX designed in Fig. 7.2 can be represented by the logic symbol shown in Fig This multiplexer acts like a digital switch in which one of the inputs c[3:0] gets connected to the output z. The switch is controlled by the two control lines s[1:0]. The two bits on these control lines select one of the four inputs to be "connected" to the output. Note that we constructed this 4-to-1 multiplexer using three 2-to-1 multiplexers in a tree fashion as shown in Fig Figure 7.5 A 4-to-1 multiplexer Recall from Eq. (5.1) in Example 5 that the logic equation for a 2-to-1 MUX is given by y = ~s & a s & b (7.1)

41 4-to-1 Multiplexer 35 will assign the value of c[2] to the output z when the input value s[1:0] is equal to 2 (binary 10). Note that the output z must be of type reg because its value is assigned within an always clause. In the case statement the alternative value preceding the colon in each line represents the value of the case parameter, in this case the 2-bit input s. These values are decimal values by default. If you want to write a hex value you precede the number with h as in ha which is a hex value A. Similarly, a binary number is preceded with a b as in b1010 which has the same value (10) as ha. Normally, binary numbers are preceded with the number of bits in the number such as 4 b107. Using this notation, the number 8 b will be the binary number Listing 7.3 mux41c.v // Example 7c: 4-to-1 MUX using case statement module mux41c ( input wire [3:0] c, input wire [1:0] s, output reg z case(s) 0: z = c[0]; 1: z = c[1]; 2: z = c[2]; 3: z = c[3]; default: z = c[0]; endcase endmodule All case statements should include a default line as shown in Listing 7.3. This is because all cases need to be covered and while it looks as if we covered all cases in Listing 7.3, Verilog actually defines four possible values for each bit, namely 0 (logic value 0), 1 (logic value 1), Z (high impedance), and X (unkown value). A simulation of the program in Listing 7.3 will produce the same result as in Fig. 7.3 (without the wire signals v and w). 7.4 A Quad 4-to-1 Multiplexer To make a quad 4-to-1 multiplexer we could combine four 4-to-1 MUXs as we did for a quad 2-to-1 multiplexer module in Fig. 6.1 of Example 6. However, it will be easier to modify the case statement program in Listing 7.3 to make a quad 4-to-1 MUX. Because we will use it in Example 10 we will define a single 16-bit input x[15:0] and we will multiplex the four hex digits making up this 16-bit value. Listing 7.4 is a Verilog program for this quad 4-to-1 multiplexer. Note that the four hex digits making up the 16-bit value of x[15:0] are multiplexed to the output z[3:0] depending of the value of the control signal s[1:0]. A simulation of this quad 4-to-1 multiplexer is shown in Fig. 7.6 and its BDE symbol is shown in Fig. 7.7.

43 Clocks and Counters 37 Example 8 Clocks and Counters The Nexys-2 board has an onboard 50 MHz clock. The BASYS board has a jumper that allows you to set the clock to 100 MHz, 50 MHz, or 25 MHz. All of the examples in this book will assume an input clock frequency of 50 MHz. If you are using the BASYS board you should remove the clock jumper, which will set the clock frequency to 50 MHz. This 50 MHz clock signal is a square wave with a period of 20 ns. The FPGA pin associated with this clock signal is defined in the constraints file basys2.ucf or nexys2.ucf with the name mclk. In this example we will show how to design an N-bit counter in Verilog and how to use a counter to generate clock signals of lower frequencies. Prerequisite knowledge: Appendix A Use of Aldec Active-HDL 8.1 N-Bit Counter The BDE symbol for an N-bit counter is shown in Fig If the input clr = 1 then all N of the outputs q[i] are cleared to zero asynchronously, i.e., regardless of the value of the input clk. If clr = 0, then on the next rising edge of the clock input clk the N- bit binary output q[n-1:0] will be incremented by 1. That is, on the rising edge of the clock the N-bit binary output q[n-1:0] will count from 0 to N-1 and then wrap around to 0. Figure 8.1 An N-bit counter The Verilog program shown in Listing 8.1 was used to generate the symbol shown in Fig Note that the sensitivity list of the always statement contains the phrase posedge clk or posedge clr This means that the if statement within the always block will execute whenever either clr or clk goes high. If clr goes high then the output q[n-1:0] will go to zero. On the other hand if clr = 0 and clk goes high then the output q[n-1:0] will be incremented by 1. The default value of the parameter N in Listing 8.1 is 4. A simulation of this 4-bit counter is shown in Fig Note that this counter counts from 0 to F and then wraps

45 Clocks and Counters 39 In the simulation in Fig. 8.2 note that the output q[0] is a square wave at half the frequency of the input clk. Similarly, the output q[1] is a square wave at half the frequency of the input q[0], the output q[2] is a square wave at half the frequency of the input q[1], and the output q[3] is a square wave at half the frequency of the input q[2]. Note how the binary numbers q[3:0] in Fig. 8.2 count from 0000 to The simulation shown in Fig. 8.2 shows how we can obtain a lower clock frequency by simply using one of the outputs q[i]. We will use this feature to produce a 24-bit clock divider in the next section. 8.2 Clock Divider The simulation in Fig. 8.2 shows that the outputs q[i] of a counter are square waves where the output q[0] has a frequency half of the clock frequency, the output q[1] has a frequency half of q[0], etc. Thus, a counter can be used to divide the frequency f of 1 a clock, where the frequency of the output q(i) is 2 i + fi = f. The frequencies and periods of the outputs of a 24-bit counter driven by a 50 MHz clock are shown in Table 8.1. Note in Table 8.1 that the output q[0] has a frequency of 25 MHz, the output q[17] has a frequency of Hz, and the output q[23] has a frequency of 2.98 Hz. Table 8.1 Clock divide frequencies q[i] Frequency (Hz) Period (ms) i

46 40 Example 8 The Verilog program shown in Listing 8.2 is a 24-bit counter that has three outputs, a 25 MHz clock (clk25), a 190 Hz clock (clk190), and a 3 Hz clock (clk3). You can modify this clkdiv module to produce any output frequency given in Table 8.1. We will use such a clock divider module in many of our top-level designs. Listing 8.2 clkdiv.v // Example 8b: clock divider module clkdiv ( input wire clk, input wire clr, output wire clk190, output wire clk25, output wire clk3 reg [23:0] q; // 24-bit counter clk or posedge clr) begin if(clr == 1) q <= 0; else q <= q + 1; end assign clk190 = q[17]; assign clk25 = q[0]; assign clk3 = q[23]; // 190 Hz // 25 MHz // 3 Hz endmodule Note in Listing 8.2 that we define the internal signal q[23:0] of type reg. It must be of type reg because its value is assigned within an always block. The BDE symbol generated by compiling Listing 8.2 is shown in Fig You can edit either Listing 8.2 or the block diagram shown in Fig. 8.3 to bring out only the clock frequencies you need in a particular design. For example, the top-level design shown in Fig. 8.4 will cause the eight LEDs on the FPGA board to count in binary at a rate of about three counts per second. The corresponding top-level Verilog program is shown in Listing 8.3. Figure 8.3 A clock divider

47 Clocks and Counters 41 Figure 8.4 Counting in binary on the eight LEDs Listing 8.3 count8_top.v // Example 8c: count8_top module count8_top ( input wire mclk; input wire [3:3] btn; output wire [7:0] ld; ) ; wire clk3; clkdiv U1 (.clk3(clk3),.clr(btn[3]),.mclk(mclk) counter #(.N(8)) U2 (.clk(clk3),.clr(btn[3]),.q(ld[7:0]) endmodule Internally, a counter contains a collection of flip-flops. We saw in Fig. 1 of the Introduction that each of the four slices in a CLB of a Spartan3E FPGA contains two flip-flops. Such flip-flops are central to the operation of all synchronous sequential circuits in which changes take place on the rising edge of a clock. The examples in the second half of this book will involve sequential circuits beginning with an example of an edge-triggered D flip-flop in Example 16.

48 42 Example 9 Example 9 7-Segment Decoder In this section we will show how to design a 7-segment decoder using Karnaugh maps and write a Verilog program to implement the resulting logic equations. We will also solve the same problem using a Verilog case statement. Prerequisite knowledge: Karnaugh maps Appendix D case statement Example 7 LEDs Example Segment Displays Seven LEDs can be arranged in a pattern to form different digits as shown in Fig Digital watches use similar 7-segment displays using liquid crystals rather than LEDs. The red digits on digital clocks are LEDs. Seven segment displays come in two flavors: common anode and common cathode. A common anode 7-segment display has all of the anodes tied together while a common cathode 7-segment display has all the cathodes tied together as shown in Fig V f g a b a b c d e f g Common Anode e c a b c d e f g Common Cathode d Figure 9.1 A 7-segment display contains seven light emitting diodes (LEDs) The BASYS and Nexys2 boards have four common-anode 7-segment displays. This means that all the anodes are tied together and connected through a pnp transistor to +3.3V. A different FPGA output pin is connected through a 100Ω current-limiting resistor to each of the cathodes, a g, plus the decimal point. In the common-anode case, an output 0 will turn on a segment and an output 1 will turn it off. The table shown in

51 7-Segment Decoder Segment Decoder: case Statement We can use a Verilog case statement to design the same 7-segment decoder that we designed in Section 9.2 using Karnaugh maps. The Verilog program shown in Listing 9.2 is a hex-to-seven-segment decoder that converts a 4-bit input hex digit, 0 F, to the appropriate 7-segment codes, a g. The case statement in Listing 9.2 directly implements the truth table in Fig Recall that a typical line in the case statement, such as 3: a_to_g = 7'b ; will assign the 7-bit binary value, , to the 7-bit array, a_to_g, when the input hex value x[3:0] is equal to 3 (0011). In the array a_to_g the value a_to_g[6] corresponds to segment a and the value a_to_g[0] corresponds to segment g. Recall that in Verilog a string of binary bits is preceded by n b, where n is the number of binary bits in the string. In the case statement the value preceding the colon in each line represents the value of the case parameter, in this case the 4-bit input x. Also note that hex values such as A are written as ha. Recall that all case statements should include a default line as shown in Listing 9.2. This is because all cases need to be covered and while it looks as if we covered all cases in Listing 6.2, Verilog actually defines four possible values for each bit, namely 0 (logic value 0), 1 (logic value 1), Z (high impedance), and X (unkown value). A simulation of Listing 9.2 will produce the same results as shown in Fig It should be clear from this example that using the Verilog case statement is often easier than solving for the logic equations using Karnaugh maps. To test the 7-segment displays on the FPGA board you could create the design hex7seg_top.bde shown in Fig This design uses the Verilog program hex7seg.v from Listing 9.2 and produces a top-level Verilog program hex7seg_top.v equivalent to Listing 9.3. Each of the four digits on the 7-segment display is enabled by one of the active low signals an[3:0] and all digits share the same a_to_g[6:0] signals. If an[3:0] = 0000 then all digits are enabled and display the same hex digit. This is what we do in Fig. 9.6 and Listing 9.3. Making the output dp = 1 will cause the decimal points to be off. You should be able to display all of the hex digits from 0 F by changing the four rightmost switches. Figure 9.6 Top-level design for testing hex7seg

53 7-Segment Displays: x7seg and x7segb 47 Example 10 7-Segment Displays: x7seg and x7segb In this example we will show how to display different hex values on the four 7- segment displays. Prerequisite knowledge: Karnaugh maps Appendix D case statement Example 7 LEDs Example Multiplexing 7-Segment Displays We saw in Example 9 that the a_to_g[6:0] signals go to all of the 7-segment displays and therefore in that example all of the digits displayed the same value. How could we display a 4-digit number such as 1234 that contains different digits? To see how we might do this, consider the BDE circuit shown in Fig Instead of enabling all four digits at once by setting an[3:0] = 0000 as we did in Fig. 9.6 we connect an[3:0] to the NOT of the four pushbuttons btn[3:0]. Thus, a digit will only be enabled when the corresponding pushbutton is being pressed. The quad 4-to-1 multiplexer, mux44, from Listing 7.4 is used to display the 16-bit number x[15:0] as a 4-digit hex value on the 7-segment displays. When you press btn[0] if the control signal s[1:0] is 00 then x[3:0] becomes the input to the hex7seg module and the value of x[3:0] will be displayed on digit 0. Similarly if you press btn[1] and the control signal s[1:0] is 01 then x[7:4] becomes the input to the hex7seg module and the value of x[7:4] will be displayed on digit 1. We can make the value of s[1:0] depend on the value of btn[3:0] using the truth table in Fig From this truth table we can write the following logic equations for s[1] and s[0]. s[1] = btn[2] btn[3]; s[0] = btn[1] btn[3]; The two OR gates in Fig will implement these logic equations for s[1:0]. btn[3] btn[2] btn[1] btn[0] s[1] s[0] X X Figure 10.1 Truth table for generating s[1:0] in Fig. 10.2

55 7-Segment Displays: x7seg and x7segb Segment Displays: x7seg We saw in Section 10.1 that to display a 16-bit hex value on the four 7-segment displays we must multiplex the four hex digits. You can only make it appear that all four digits are on by multiplexing them fast enough (greater than 30 times per second) so that your eyes retain the values. This is the same way that your TV works where only a single picture element (pixel) is on at any one time, but the entire screen is refreshed 30 times per second so that you perceive the entire image. To do this the value of s[1:0] in Fig must count from 0 to 3 continually at this fast rate. At the same time the value of the outputs an[3:0] must be synchronized with s[1:0] so as to enable the proper digit at the proper time. A circuit for doing this is shown in Fig The outputs an[3:0] will satisfy the truth table in Fig Note that each output an[i] is just the maxterm M[i] of q[1:0]. Figure 10.3 BDE circuit x7seg.bde for displaying x[15:0] on the four 7-segment displays q[1] q[0] an[3] an[2] an[1] an[0] Figure 10.4 Truth table for generating an[3:0] in Fig A simulation of x7seg.bde is shown in Fig Note how the an[3:0] output selects one digit at a time to display the value 1234 on the 7-segment displays. When x7seg.bde is compiled it creates a Verilog program that is equivalent to Listing The top-level design shown in Fig can be used to test the x7seg module on the FPGA board. The Verilog program corresponding to this top-level design is given in Listing Note that the x7seg module requires a 190 Hz clock generated by the clock divider module clkdiv from Example 8.

58 52 Example 10 called x7segb, that displays leading zeros as blanks is shown in Listing This is done by writing logic equations for aen[3:0] that depend on the values of x[15:0]. For example, aen[3] will be 1 (and thus digit 3 will not be blank) if any one of the top four bits of x[15:0] is 1. Similarly, aen[2] will be 1 if any one of the top eight bits of x[15:0] is 1, and aen[1] will be 1 if any one of the top twelve bits of x[15:0] is 1. Note that aen[0] is always 1 so that digit 1 will always be displayed even if it is zero. To test the module x7segb you can run the top-level design shown in Listing 10.4 that will display the value of x on the 7-segment displays where x is defined by the following statement: assign x = {sw,btn[2:0],5'b01010}; // digit 0 = A The curly brackets {--,--} are used for concatenation in Verilog. In this case we form the 16-bit value of x by concatenating the eight switches, the three right-most pushbuttons, and the five bits Note that if all switches are off an A will be displayed on digit 0 with three leading blanks. Turning on the switches and pushing the three right-most pushbuttons will display various hex numbers always with leading blanks. Listing 10.4 x7segb.v // Example 10d: x7segb - Display 7-seg with leading blanks // input cclk should be 190 Hz module x7segbc ( input wire [15:0] x, input wire cclk, input wire clr, output reg [6:0] a_to_g, output reg [3:0] an, output wire dp reg [1:0] s; reg [3:0] digit; wire [3:0] aen; assign dp = 1; // set aen[3:0] for leading blanks assign aen[3] = x[15] x[14] x[13] x[12]; assign aen[2] = x[15] x[14] x[13] x[12] x[11] x[10] x[9] x[8]; assign aen[1] = x[15] x[14] x[13] x[12] x[11] x[10] x[9] x[8] x[7] x[6] x[5] x[4]; assign aen[0] = 1; // digit 0 always on // Quad 4-to-1 MUX: mux44 case(s) 0: digit = x[3:0]; 1: digit = x[7:4]; 2: digit = x[11:8]; 3: digit = x[15:12]; default: digit = x[3:0]; endcase

61 2's Complement 4-Bit Saturator 55 Example 11 2's Complement 4-Bit Saturator In this example we will design a circuit that converts a 6-bit signed number to a 4- bit output that gets saturated at -8 and +7. Prerequisite knowledge: Basic Gates Appendix C Equality Detector Example 6 Quad 2-to-1 Multiplexer Example 6 7-Segment Displays Example Creating the Design sat4bit.bde Figure 11.1 shows a circuit called sat4bit.bde that was described in the November 2001 issue of NASA Tech Briefs. The circuit will take a 6-bit two s complement number with a signed value between 32 and +31 and convert it to a 4-bit two s complement number with a signed value between 8 and +7. Negative input values less than 8 will be saturated at 8. Positive input values greater than +7 will be saturated at +7. Note that the two XNOR gates and the AND gate form an equality detector whose output s is 1 when x[3], x[4], and x[5] are all equal (see Example 4). This will be the case when the 6-bit input number x[5:0] is between -8 and +7. In this case output y[3:0] of the quad 2-to-1 MUX will be connected to the input x[3:0]. If the top three bits of x[5:0] are not equal and x[5] is 1 then the input value will be less than -8 and the output y[3:0] of the quad 2-to-1 MUX will be saturated at -8. On the other hand if the top three bits of x[5:0] are not equal and x[5] is 0 then the input value will be greater than +7 and the output y[3:0] of the quad 2-to-1 MUX will be saturated at +7. Figure 11.1 Circuit diagram for sat4bit.bde

62 56 Example 11 Listing 11.1 sat4bit.v // Example 11a: sat4bit module sat4bit ( input wire [5:0] x, output wire [3:0] y wire c0; wire c1; wire s; wire xi; assign c1 = ~(x[4] ^ x[3] assign xi = ~(x[5] assign c0 = ~(x[5] ^ x[4] assign s = c0 & c1; mux24 U1 (.a({x[5],xi,xi,xi}),.b(x[3:0]),.s(s),.y(y) endmodule A top-level design that can be used to test sat4bit is shown in Fig The module x7segb11 is a modification of Listing 10.4 that will display only values between -8 and +7 on the 7-segment display. Listing 11.2 shows the Verilog program for the module x7segb11. The input to x7segb11 is the 4-bit output y[3:0] from sat4bit. Note that only the two rightmost 7-segment display are enabled. The two leftmost displays are always blank. The hex7seg always block in Listing 11.2 has been modified to display the magnitude of the signed value of y[3:0] 0 to 8. The preceding 7-segment display will either be blank or display a minus sign. The quad 4-to-1 MUX and the new 2-to-1 MUX are used to display the minus sign when aen[1] is enabled if y[3] is 1; i.e., if y is negative. Figure 11.2 Top-level design sat4bit_top.bde for testing sat4bit

66 60 Example 12 Example 12 Full Adder In this example we will design a full adder circuit. Prerequisite knowledge: Basic Gates Appendix C Karnaugh Maps Appendix D 7-Segment Displays Example Half Adder The truth table for a half adder is shown in Fig In this table bit a is added to bit b to produce the sum bit s and the carry bit c. Note that if you add 1 to 1 you get 2, which in binary is 10 or 0 with a carry bit. The BDE logic diagram, halfadd.bde, for a half adder is also shown in Fig Note that the sum s is just the exclusive-or of a and b and the carry c is just a & b. The Verilog program corresponding to the circuit in Fig is shown in Listing A simulation of halfadd.bde is shown in Fig a b s c Figure 12.1 Truth table and logic diagram halfadd.bde for a half-adder Listing 12.1 halfadd.v // Example 12a: halfadd module halfadd ( input wire a, input wire b, output wire c, output wire s ) ; assign s = b ^ a; assign c = b & a; endmodule

67 Full Adder 61 Figure 12.2 Simulation of the half-adder in Fig Full Adder When adding binary numbers we need to consider the carry from one bit to the next. Thus, at any bit position we will be adding three bits: a i, b i and the carry-in c i from the addition of the two bits to the right of the current bit position. The sum of these three bits will produce a sum bit, s i, and a carry-out, c i+1, which will be the carry-in to the next bit position to the left. This is called a full adder and its truth table is shown in Fig The results of the first seven rows in this truth table can be inferred from the truth table for the half adder given in Fig In all of these rows only two 1's are ever added together. The last row in Fig adds three 1's. The result is 3, which in binary is 11, or 1 plus a carry. From the truth table in Fig we can write a sum of products expression for s i as c i a i b i s i c i s i = ~c i & ~a i & b i ~c i & a i & ~b i (12.1) c i & ~a i & ~b i c i & a i & b i Figure 12.3 Truth table for a full adder We can use the distributive law to factor out ~c i from the first two product terms and c i from the last two product terms in Eq. (12.1) to obtain s i = ~c i & (~a i & b i a i & ~b i ) c i & (~a i & ~b i a i & b i ) (12.2) which can be written in terms of XOR and XNOR operations as which further reduces to s i = ~c i & (a i ^ b i ) c i & ~(a i ^ b i ) (12.3)

68 62 Example 12 s i = c i ^ (a i ^ b i ) (12.4) Fig shows the K-map for c i+1 from the truth table in Fig The map shown in Fig. 12.4a leads to the reduced form for c i+1 given by c i+1 = a i & b i c i & b i c i & a i (12.5) While this is the reduced form, a more convenient form can be written from Fig. 12.4b as follows: c i+1 = a i & b i c i & ~a i & b i c i & a i & ~b i = a i & b i c i & (~a i & b i a i & ~b i ) = a i & b i c i & (a i ^ b i ) (12.6) b i a i b i c 00 i b i ab i i c 00 i c i c i (a) a i From Eqs. (12.4) and (12.6) we can draw the logic diagram for a full adder as shown in Fig Comparing this diagram to that for a half adder in Fig it is clear that a full adder can be made from two half adders plus an OR gate as shown in Fig (b) Figure 12.4 K-maps for c i+1 for full adder in Fig. 6.2 a i c i a i bi s i c i+1 Figure 12.5 Logic diagram for a full adder c i s s i a i b i half-adder s c half-adder c c i+1 Figure 12.6 A full adder can be made from two half adders plus an OR gate

70 64 Example 12 Figure 12.8 Simulation of the full adder in Fig and Listing 12.2

71 4-Bit Adder 65 Example 13 4-Bit Adder In this example we will design a 4-bit adder. Prerequisite knowledge: Basic Gates Appendix C Karnaugh Maps Appendix D Full Adder Example Bit Adder Four of the full adders in Fig can be combined to form a 4-bit adder as shown in Fig Note that the full adder for the least significant bit will have a carryin of zero while the remaining bits get their carry-in from the carry-out of the previous bit. The final carry-out, is the cout for the 4-bit addition. The Verilog program corresponding to the 4-bit adder in Fig is given in Listing Figure 13.1 Block diagram adder4.bde for a 4-bit adder

72 66 Example 13 Listing 13.1 adder4.v // Example 13a: adder4 module adder4 ( input wire cin; input wire [3:0] a; input wire [3:0] b; output wire cout; output wire [3:0] s; ) ; wire c1; wire c2; wire c3; fulladd U1 (.a(a[2]),.b(b[2]),.cin(c2),.cout(c3),.s(s[2]) fulladd U2 (.a(a[3]),.b(b[3]),.cin(c3),.cout(cout),.s(s[3]) fulladd U3 (.a(a[1]),.b(b[1]),.cin(c1),.cout(c2),.s(s[1]) fulladd U4 (.a(a[0]),.b(b[0]),.cin(cin),.cout(c1),.s(s[0]) endmodule A simulation of the 4-bit adder in Fig and Listing 13.1 is shown in Fig The value of a is incremented from 0 to F and is added to the hex value B. The sum s is always equal to a + b. Note that the carry flag, cout, is equal to 1 when the correct unsigned answer exceeds 15 (or F). We can test the adder4 module from Fig and Listing 13.1 on the FPGA board by combining it with the x7segb module from Listing 10.4 in Example 10 and the clkdiv module from Listing 8.2 from Example 8 to produce the top-level design shown in Listing The 4-bit number sw[7:4] will be displayed on the first (left-most) 7-

74 68 Example 14 Example 14 N-Bit Adder In this example we will design a N-bit adder. Prerequisite knowledge: 4-Bit Adder Example Bit Adder: Behavioral Statements It would be convenient to be able to make a 4-bit adder (or any size adder) by just using a + sign in a Verilog statement. In fact, we can! When you write a + b in a Verilog program the compiler will produce a full adder of the type we designed in Example 12. The only question is how to create the output carry bit. The trick is to add a leading 0 to a and b and then make a 5-bit temporary variable to hold the sum as shown in Listing The most-significant bit of this 5-bit sum will be the carry flag. A simulation of this program is shown in Fig Compare this with Fig Listing 14.1 adder4b.v // Example 14a: 4-bit behavioral adder module adder4b ( input wire [3:0] a, input wire [3:0] b, output reg [3:0] s, output reg cf reg [4:0] temp; begin temp = {1'b0,a} + {1'b0,b}; s = temp[3:0]; cf = temp[4]; end endmodule Figure 14.1 Simulation of the Verilog program in Listing 14.1

75 N-Bit Adder N - Bit Adder: Behavioral Statements Listing 14.2 shows an N-bit adder that uses a parameter statement. This is a convenient adder to use when you don t need the carry flag. An example of using this as an 8-bit adder is shown in the simulation in Fig Note that when the sum exceeds FF it simply wraps around and the carry flag is lost. Listing 14.2 adder.v // Example 14b: N-bit adder module adder #(parameter N = 8) (input wire [N-1:0] a, input wire [N-1:0] b, output reg [N-1:0] y begin y = a + b; end endmodule Figure 14.2 Simulation of the Verilog program in Listing 14.2 The top-level design shown in Fig can be used to test this N-bit adder on the FPGA board. In this case we are adding two 4-bit switch settings and observing the sum on the 7-segment display. To set the parameter N to 4 right-click on the adder symbol, select Properties and click on the Parameter tab. Set the actual value of N to 4. Figure 14.3 Top-level design for testing the N-bit adder on the FPGA board

76 70 Example 15 Example 15 N-Bit Comparator In this example we will design a N-bit comparator. Prerequisite knowledge: N-Bit Adder Example N-Bit Comparator Using Relational Operators The easiest way to implement a comparator in Verilog is to use the relational and logical operators shown in Table An example of using these to implement an N-bit comparator is shown in Listing A simulation of this program for the default value of N = 8 is shown in Fig Note in the always block in Listing 15.1 we set the values of gt, eq, and lt to zero before the if statements. This is important to make sure that each output has a value assigned to it. If you don t do this then Verilog will assume you don t want the value to change and will include a latch in your system. Your circuit will then not be a combinational circuit. Table 15.1 Relational and Logical Operators Operator Meaning == Logical equality!= Logical inequality < Less than <= Less than or equal > Greater than >= Greater than or equal! Logical negation && Logical AND Logical OR Figure 15.1 Simulation of the Verilog program in Listing 15.1

77 N-Bit Comparator 71 Listing 15.1 comp.v // Example 17: N-bit comparator using relational operators module comp #(parameter N = 8) (input wire [N-1:0] x, input wire [N-1:0] y, output reg gt, output reg eq, output reg lt begin gt = 0; eq = 0; lt = 0; if(x > y) gt = 1; if(x == y) eq = 1; if(x < y) lt = 1; end endmodule You can test this comparator on the FPGA board by creating the BDE block diagram comp4_top.bde shown in Fig To make this a 4-bit comparator right-click on the comp symbol, select Properties, click on the Parameters tab, and set the actual value of N to 4. You will be comparing the 4-bit number x[3:0] on the left four switches with the 4-bit number y[3:0] on the right four switches. The three LEDs ld[4:2] will detect the outputs gt, eq, and lt. We selected these three LEDs because on the BASYS board they are three different colors. Compile the design comp4_top.bde, implement it, and download the.bit file to the FPGA board. Test the comparator by changing the switch settings. Figure 15.2 Top-level design comp4_top.bde to test a 4-bit comparator

78 72 Example 15

79 Aldec Active-HDL Tutorial 109 Appendix A Aldec Active-HDL Tutorial Part 1: Project Setup Start the program by double-clicking the Active-HDL icon on the desktop. Select Create new workspace and click OK. Browse to the directory where you want the project saved, type Example1 for the workspace name and click OK.

85 Aldec Active-HDL Tutorial 115 This will generate a block diagram (schematic) template with the input and output ports displayed. You will need to select the output port by dragging the mouse with the left mouse button down and move the output port to the left. Select the bus icon and connect the input sw[7:0] to the output ld[7:0] as shown. Click Save

90 120 Appendix A Click implementation When implementation is complete click Close. Part 4: Program FPGA Board To program the Spartan3E on the BASYS or Nexys-2 boards we will use the ExPort tool that is part of the the Adept Suite available free from Digilent at Double-click the ExPort icon on the desktop. Click Initialize Chain

91 Aldec Active-HDL Tutorial 121 Click Browse and go to Example1->swled->implement->ver1->rev1->sw2led.bit Select sw2led.bit Click Program Chain Your program is now running on the board. Change the switches and watch the LEDs.

95 Aldec Active-HDL Tutorial 125 This will generate a block diagram (schematic) template with the input and output ports displayed. Select the output ports by dragging the mouse with the left mouse button down and move the output ports to the left. Click the Show Symbols Toolbox icon Click + on Built-in symbols

96 126 Appendix A Grab the and2 symbol with the mouse and drag it to the output port and_ Grab the symbols for nand2, or2, nor2, xor2, and xnor2 and drag them to the appropriate output port, moving the output ports down as necessary.

97 Aldec Active-HDL Tutorial 127 Select the wire icon and connect the gate inputs to a and b as shown. Click Save Right-click on gates2.bde and select Compile

100 130 Appendix A The waveform window will automatically come up with the simulation already initialized. Make sure the order is a, b, and_, nand_, or_, nor_, xor_, xnor (grab and drag if necessary). Right-click on a and select Stimulators. Select Clock and set Frequency to 25 MHz Click Apply

101 Aldec Active-HDL Tutorial 131 Click on b, select Clock and set Frequency to 50 MHz Click Apply Click Close Set simulation time to 200 ns Click here to run simulation Click Zoom to Fit.

104 134 Appendix A This will generate a Verilog template with the input and output signals filled in. Delete all the comments and replace them with the single comment // Example 1: 2-input gates Delete these statements. Edit the module, input, output, and wire statements to conform to the 2001 Verilog standard as shown (see Listing 2.1 in Example 1). Delete these comments.

107 Aldec Active-HDL Tutorial 137 Click functional simulation The waveform window will automatically come up with the simulation already initialized. Make sure the order is a, b, z (grab and drag if necessary). Right-click on a and select Stimulators.

108 138 Appendix A Select Clock and set Frequency to 25 MHz Click Apply Click on b, select Clock and set Frequency to 50 MHz Click Apply Click Close

109 Aldec Active-HDL Tutorial 139 Click here to run simulation Set simulation time to 50 ns Click + sign to show all elements of z. Study the waveforms for various magnifications. To print out this waveform you can detach it by clicking >> here and then press Alt Prnt Scrn to copy it to the clipboard. Then paste it in a.doc file and print.

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